With my NAS, I pay for the box, install the drives, and it Just Works with basically no maintenance other than me clicking the upgrade button every few months when it emails me that an OS update is ready.

I could build a similar system myself, but the hardware isn't that much cheaper. Cases, PSUs, hot swappable drive mounts, and all that add up quickly. And when I'm done, I have to install the OS, read the docs to see how the recommended configuration has changed since last time I did this, and periodically log in to look at it because it doesn't have nearly as much monitoring set up out-of-the-box.

Given the choice between the small amount of money I'd save vs the amount of time I'd have to invest in it, I'd rather pay the money and outsource the hassle to the NAS maker.

As to why I don't just hang a bunch of drives off the computer I'm already using:

- Backups. If my Mac dies, I can restore it from the Time Machine on my NAS.

- Noise. The NAS has busier fans than my Mac. It's in a different room from where I spend most of my time.

- I run Docker stuff on it. I don't want those things running 24/7 on my desktop.

- Availability. I want to reboot my desktop occasionally without worrying if it'd interrupt my wife watching something on Plex.